Fuel injectors of an internal combustion engine have to supply a fuel to an engine system in response to operating conditions thereof. It is necessary to keep an air-to-fuel ratio in a narrow area near the stoichiometric ratio, i.e. a target ratio near the stoichiometric ratio, so that a three-way catalytic converter can effectively purify exhaust gases.
In the internal combustion engine, the air-to-fuel ratio depends upon loads and engine speeds. As shown in FIG. 11 of the accompanying drawings, the target air-to-fuel ratio should be determined depending upon whether the engine is operating with an air-to-fuel ratio which is for a fuel cutting zone, a lean zone, a stoichiometric zone or a high acceleration operating zone. There are proposed engines which mainly operate with a lean air-fuel mixture so as to save the fuel.
The air-to-fuel ratio of such an engine is usually set between a target value and the stoichiometric ratio according to the engine operating conditions. In addition, if the target air-to-fuel ratio is extensively variable in the rich and lean zones from the stoichiometric ratio, an exhaust gas purifier has to include not only a three-way catalytic converter but also a catalyst for effectively purifying NOx in lean exhaust gases. Such a catalyst is disposed before the three-way catalytic converter so as to remove NOx from the lean exhaust gases. One of such engines is exemplified in Japanese Patent Laid-Open Publication Sho 60-125250 (1985).
To feedback control this engine, it is essential to obtain data on the air-to-fuel ratio which is extensively variable in the entire engine operating zone. Wide-range air-to-fuel ratio sensors are employed for this purpose. One of such sensors is disclosed in the Japanese Patent Laid-Open Publication Hei 2-204326 (1991).
A control unit for this purpose calculates a corrective air-to-fuel ratio based on actual air-to-fuel ratio data measured by the wide range air-to-fuel ratio sensor and a target air-to-fuel ratio (in the rich and lean zones from the stoichiometric ratio) which is set for a possible engine operating condition. The corrective air-to-fuel ratio removes the deviation of the actual air-to-fuel ratio from the target air-to-fuel ratio. Then, the amount of fuel to be injected is calculated to satisfy the corrective air-to-fuel ratio, so that fuel injectors will deliver the calculated amount of the fuel.
The present invention aims at solving the following problems of conventional air-to-fuel ratio control systems.
When an air-to-fuel ratio sensor or a fuel injector becomes out of use in any of the foregoing air-to-fuel control systems, the air-to-fuel ratio would be erroneously corrected in the feedback control process, with unreliable operation or interruption of the engine being caused, or the engine being damaged due to knocking.
The foregoing inconveniences may be solved by uniformly setting the maximum and minimum allowable ranges of the corrective value in the feedback control. However, since the feedback control capability per step is limited, the air-to-fuel ratio sometimes has to be controlled in a plurality of steps.
With the foregoing prior problems in view, it is an object of the invention to provide an air-to-fuel ratio control system which can effectively prevent over-correction of the air-to-fuel ratio in the feedback control process.